Field of the Invention
The present invention relates to heat exchangers of the type used for transmitting heat from one fluid flow to another. The fluid flows may be both liquid or both gaseous, one liquid and the other gaseous, or one or both flows might be a mixture of liquid and gas.
Heat exchangers are of considerable importance in many manufacturing processes and in many manufactured goods. A continual problem with the design of heat exchangers is the compromise between efficiency and robustness. Efficiency is, in general, improved by using thinner primary plates made up into tubes or ducts of small cross-section (a primary plate being a plate directly separating two different fluid streams). However this often leads to fragility. Undue fragility is unacceptable for many uses of heat exchangers--for example in motor vehicles. It is therefore common practice to use secondary plates in heat exchangers to improve the heat exchangeability, the strength or both.
A typical form of secondary plate consists of a series of fins extending into or through one fluid flow stream and bonded to one or more primary plates dividing that fluid flow stream from one or more flow streams of the other fluid. One example of a finned arrangement is described in U.S. Pat. No. 2,471,582 where one fluid passes through a tube which has applied to its outer surface at least one heat transfer fin formed from the material known as expanded metal. Expanded metal is a well-known engineering material and consists of a mesh produced by forming a plurality of slits in a metal plate and expanding the plate. This type of heat exchanger is of necessity fairly bulky. Also the means whereby the fins are bonded to the primary surface, such as brazing, can limit the materials available and can give rise to corrosion problems. Flow streams can be in crossflow or in counterflow, and in the latter case special distributor sections can be required to achieve uniform flow.
A more recent invention, offering greater compactness and range of construction materials, is the Printed Circuit Heat Exchanger or PCHE, (U.S. Pat. No. 4,665,975), in which flat plates are photochemically etched with heat-transfer passages and then diffusion bonded together to form a solid block. This can operate at very high temperatures and pressures. As with the plate-fin heat exchanger, the flow streams can be in either cross or counterflow. The plates in this heat exchanger, however, are all primary, leading to an inefficient use of material for many purposes such as gas flows.
The use of secondary plates raises its own problems, as it inevitably results in greater complexity, and extra volume. The extra volume is undesirable, as space is usually a major factor in industrial conditions. There is therefore a need for heat exchangers having secondary plates providing improved heat transfer properties and increased strength without an inordinate increase in size.